Adjustable bit rate optical transmission using programmable signal modulation

ABSTRACT

Techniques, apparatus and systems to provide adjustable bit rate optical transmission using programmable signal modulation in optical communication systems.

BACKGROUND

This application relates to optical communications.

Optical communications use an optical modulator to modulate an opticalcarrier beam to carry digital bits for transmission over an opticallink. An optical communication system can use optical wavelengthdivision multiplexing (WDM) to transmit multiple optical carriersmodulated to carry different optical data channels through a singlefiber.

The performance of optical transmission can be characterized by variousparameters, such as the optical signal to noise ratio (OSNR), the databit error rate (BER) and the data bit rate per wavelength or dataspectral efficiency. The signal quality of an optical WDM signal may bedegraded by various effects in the optical transmission such as opticalattenuation effects in fiber and optical dispersion effects includingchromatic dispersion (CD), polarization mode dispersion (PMD) andpolarization dependent loss (PDL) in fiber. Some techniques to mitigatedegradation of optical signals in transmission use optical compensationdevices in the optical transmission paths such as optical amplifiersagainst signal attenuation and dispersion compensation devices. Othertechniques use various signal modulation techniques to generatemodulated data formats that can tolerate signal degradation effects inoptical transmission such as the fiber dispersion.

SUMMARY

This application describes, among others, techniques, apparatus andsystems to provide adjustable bit rate optical transmission usingprogrammable signal modulation in optical communication systems. In oneaspect, a method for optical communications includes operating anoptical transmitter to provide a plurality of different signalmodulation formats with different data bit rates in controlling signalmodulation in generating an optical channel signal; operating theoptical transmitter to select one signal modulation format from theplurality of different signal modulation formats to control the signalmodulation based on a condition of an optical transmission link thattransmits the optical channel signal; and operating the opticaltransmitter to select another signal modulation format from theplurality of different signal modulation formats to control the signalmodulation when the condition of the optical transmission link changes.

One implementation of the above method for optical communicationsincludes operating a programmable optical transmitter to provide aplurality of different quadrature amplitude modulation (QAM)constellations with different data bit rates in controlling signalmodulation of an optical channel signal with a variable data bit rateselected from the QAM constellations; operating the optical transmitterto select one of the QAM constellations to control the signal modulationbased on a condition of an optical transmission link that transmits theoptical channel signal; and operating the optical transmitter to selectanother one of the QAM constellations to control the signal modulationwhen the condition of the optical transmission link changes.

In another aspect, a system for optical communications includes anoptical transmitter comprising a digital signal processing unit that isprogrammed to include a plurality of different signal modulation formatswith different data bit rates in controlling signal modulation ingenerating an optical channel signal; an optical transmission link incommunication with the optical transmitter to transmit the opticalchannel signal; an optical receiver in communication with the opticaltransmission link to receive the optical channel signal from the opticaltransmitter; and a feedback mechanism that communicates to the opticaltransmitter a feedback signal indicative of a condition of the opticaltransmission link in transmitting the optical channel signal form theoptical transmitter to the optical receiver. The optical transmitterresponds to the feedback signal to select one signal modulation formatfrom the plurality of different signal modulation formats to control thesignal modulation based on the condition and selects another signalmodulation format to control the signal modulation when the condition ofthe optical transmission link changes.

In one implementation, a system for optical communications includes anoptical transponder comprising a plurality of programmable opticaltransmitters to produce optical WDM channel signals at different opticalWDM wavelengths. Each programmable optical transmitter comprises adigital signal processing unit that is programmed to include a pluralityof different quadrature amplitude modulation (QAM) constellations withdifferent data bit rates in controlling signal modulation of an opticalWDM channel signal with a variable data bit rate selected from the QAMconstellations. This system includes an optical transmission networkconnected to optical transponder to transmit the optical WDM channelsignals, at least one optical receiver in the optical transmissionnetwork to receive at least one of the optical WDM channel signals fromthe optical transponder and comprising a coherent QAM detectionmechanism to extract data from the received optical WDM channel signal;and a feedback mechanism in the optical transmission network tocommunicate to the optical transponder a feedback signal indicative of acondition of an optical transmission link that transmits the at leastone optical WDM channel signal form the optical transponder to theoptical receiver. In this system, the optical transponder responds tothe feedback signal to select one QAM constellation from the QAMconstellations to control the signal modulation in a respectiveprogrammable optical transmitter based on the feedback signal andselects another QAM constellation to control the signal modulation inthe respective programmable optical transmitter when the condition ofthe optical transmission link changes.

In yet another aspect, a method for optical communications includesconnecting programmable optical transponder in an optical communicationnetwork, where each programmable optical transponder comprises aplurality of programmable optical transmitters to produce optical WDMchannel signals at different optical WDM wavelengths and a plurality ofoptical receivers for detecting optical WDM channel signals. Eachprogrammable optical transmitter comprises a digital signal processingunit that is programmed to include a plurality of different quadratureamplitude modulation (QAM) constellations with different data bit ratesin controlling signal modulation of an optical WDM channel signal with avariable data bit rate selected from the plurality of the QAMconstellations. This method includes obtaining performance informationfor each optical path link for each of the optical WDM channel signalsproduced by a programmable optical transponder; operating each of theprogrammable optical transmitters in each optical transponder under aselected QAM constellation that is selected from the plurality of theQAM constellations in the digital processing unit of the programmableoptical transmitter based on the performance information for therespective optical path link; providing a feedback mechanism in theoptical network to communicate to the optical transponder a feedbacksignal indicative of a change of the performance of the optical pathlink for each optical WDM channel signal from a respective programmableoptical transmitter; and operating a programmable optical transmitter tochange a selected QAM constellation currently in use to a different QAMconstellation when the respective change of the performance of theoptical path link meets a pre-determined condition for changing the QAMconstellation.

In yet another aspect, a method for optical communications includesproviding a plurality of programmable optical transmitters in an opticalnode in a network. Each programmable optical transmitter includes aplurality of different quadrature amplitude modulation (QAM)constellations with different data bit rates and operates to controlsignal modulation of an optical channel signal with a variable data bitrate selected from the QAM constellations. This method includesdetermining optical transmission performance of optical path links fortransmitting optical channel signals from the optical transmitters inthe optical node, respectively, based on at least one of an optical pathlink length and an optical signal to noise ratio for each of the opticalpath links to select one of the QAM constellations for each opticaltransmitter to control the signal modulation; and operating the opticaltransmitters in the optical node under the selected QAM constellationswith different data bit rates.

These and other implementations and their variations are described indetail in the attached drawings, the detailed description and theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an example of an optical communication system thatimplements the present adjustable bit rate optical transmission usingprogrammable signal modulation.

FIG. 1B illustrates an optical transponder in the system in FIG. 1A withprogrammable adjustable bit rate optical transmitters and correspondingoptical receivers for different optical WDM wavelengths.

FIG. 1C illustrates an exemplary adjustable bit rate optical transmitterusing programmable signal modulation and an exemplary optical receiver,respectively.

FIG. 2 shows examples of M-ary QAM constellations and representativeOSNRs, DAC and ADC bit resolution requirements, bit rate and spectralefficiencies a baud rate of 25 Gb suitable for transmission through 50GHz spaced reconfigurable optical add and drop modules (ROADMs) in denseWDM (DWDM) communication systems.

FIG. 3 shows M-QAM bit rate vs. OSNR tradeoff.

FIG. 4 shows a networking example for the M-QAM optimization perwavelength channel. The link path 410 between ROADM1 and ROADM3 has areach of 1,400 km and is based on a PM-16QAM modulation with a capacityof 200 Gb/s. The link path 420 between ROADM1 and ROADM2 has a reach of400 km and is based on a PM-64QAM modulation with a capacity of 300Gb/s. The link path 430 between ROADM3 and ROADM4 has a reach of 1,000km and is based on a PM-32QAM modulation with a capacity of 250 Gb/s.The link path 440 between ROADM2 and ROADM5 has a reach of 2,600 km andis based on a PM-8QAM modulation with a capacity of 150 Gb/s. The linkpath 450 between ROADM1 and ROADM5 has a reach of 3,000 km and is basedon a PM-QPSK modulation with a capacity of 100 Gb/s.

FIG. 5 shows an example of rate adjustable optical terminal in anoptical network having ROADMs and EDFAs.

FIGS. 6, 7 and 8 illustrate examples of three different modes ofoperating the present adjustable bit rate optical transmission usingprogrammable signal modulation.

FIG. 9 shows an example of an optical system based on adaptive bit rateoptical transmission 110 using programmable signal modulation anddynamic feedback.

FIG. 10 shows an example of the system in FIG. 9 for control of adaptivebite rate transponder using pre-FEC BER data from receiver communicatedvia “in-band” General Communications Channel (GCC) to transmitter to setoptimum bit rate.

FIG. 11 shows an example for monitoring of system margin (derived frompre-FEC BER) used to set optimum M-QAM constellation in the system inFIG. 10. Points A-D represent four different points. At point A, a lowmargin threshold is crossed, and the number of levels in the QAMconstellation is decreased. At point B, the performance margin increasesas the number of levels in the QAM constellation is decreased. At pointC, the performance margin decreases as the number of levels in the QAMconstellation is increased. At point D, a high margin threshold iscrossed and the number of levels in the QAM constellation is increased.

DETAILED DESCRIPTION

Examples of techniques, apparatus and systems described in thisapplication provide adjustable bit rate optical transmission usingprogrammable signal modulation at an optical transmitter. As an example,a digital signal processing unit can be provided to control the opticalmodulation in the optical transmitter so that different data modulationformats with different data bit rates, different data spectralefficiencies and different OSNR sensitivity levels can be used in theoptical transmitter based on the system requirements on the specificoptical WDM signal of the optical transmitter. Therefore, a singleoptical transmitter can be operated in an adjustable manner, in responseto versatile operating conditions and system requirements, to maintain adesired level of transmission performance or optimize the transmissionperformance without the need for replacing the optical transmitter. Suchoptical transmitters and respective optical receivers can also be usedto upgrade optical communication systems with higher bandwidths andimproved spectral efficiency by replacing inadequate opticaltransmitters and optical receivers without replacing the opticalbackbone structures of the networks.

FIG. 1A illustrates an example of an optical communication system thatimplements the present adjustable bit rate optical transmission usingprogrammable signal modulation. An adjustable bit rate opticaltransmitter 110 is shown to be connected in an optical transmission linkor network 120 to send an optical WDM signal to one or more opticalreceivers 130 and 140. The adjustable bit rate optical transmitter 110includes a digital signal processing (DSP) unit that is programmed tohave different signal modulation formats with different data bit ratesand controls the signal modulation format of the output optical WDMsignal from the transmitter 110. Various quadrature amplitude modulation(QAM) with different levels (M) of modulation and different data bitrates, for example, can be programmed in the digital signal processingunit of the transmitter 110. Different M-QAM constellations with a fixedbaud rate can be used.

In various optical communication systems, such an adjustable bit rateoptical transmitter 110 can be part of an optical transponder that alsoincludes an optical receiver for receiving an optical WDM signal. Theadjustable bit rate optical transmitter 110 operates to adjust itssignal modulation format selected from multiple signal modulationformats with different data bit rates based on the optical transmissionrequirements for its output optical WDM channel. FIG. 1B illustrates anoptical transponder 110TR connected to the optical link or network 120with two adjustable bit rate optical transmitters 111TX and 112TXoperating at two different optical WDM channel wavelengths for twodifferent optical links 101 and 102 in the network 120, respectively. Inthis example, two optical receivers 111RX and 112RX are paired with thetransmitters 111TX and 112TX, respectively. Each optical receiver canimplement a coherent QAM detection scheme for detecting an optical WDMchannel signal modulated under one of the QAM constellations.

In one implementation, the digital signal processing unit in the opticaltransmitter 110 can provide a programmable modulation format thatoptimizes the data bit rate per wavelength (i.e. spectral efficiency)based on the optical transmission requirements and conditions of a givenoptical link. Referring back to FIG. 1A, the optical transmitter 110 isshown to transmit over one of the two different optical path links 121and 122. Depending on which of the two optical path links 121 and 122,the signal modulation format and the associated data bit rate can beselected to enhance the performance of the signal transmission for theselected optical path link. Therefore, a single adjustable transmittercan be used to operate under different signal modulation formats withdifferent data bit rates under different transmission conditions for thelink. In some applications, for example, a network router in the network120 can switch the optical WDM channel signal from the opticaltransmitter 110 from the first optical link 121 to a first opticalreceiver 130 to a second, different optical receiver 140 at anotherlocation via the optical link 122. Because the operating conditions ofthe two optical links 121 and 122 can be different, the opticaltransmitter 110 can be controlled to adjust the QAM constellation in itssignal modulation to optimize the performance in transmitting data.

Consider a short optical transmission link that can tolerate arelatively high OSNR, a signal modulation format can be selected for theoptical transmitter to transmit at a relatively high data rate perwavelength to achieve an acceptable data error rate at the opticalreceiver. For a long transmission link that can tolerate a lower OSNR, adifferent signal modulation format can be selected to transmit at alower data rate to achieve an equivalent, acceptable error rate. Inoperation, the optical WDM signal from a particular optical transmittercan be routed to different destinations with different opticaltransmission lengths. The present adjustable bit rate opticaltransmission using programmable signal modulation can change the signalmodulation format of the optical transmitter when the routing of theoptical signal changes to optimize the transmission performance.

Notably, an optical transport network (OTN) that has many DenseWavelength Division Multiplexed (DWDM) optical channels 910 and opticalnodes with Reconfigurable Optical Add/Drop Multiplexers (ROADMs),different optical DWDM channels at different WDM wavelengths may be usedto connect different destinations such as cities and thus may havedifferent reach requirements. One way to deploy the optical DWDMtransponders is to design the transponders with fixed signal modulationformats at fixed data bit rates to support the desired transmissionperformance for the longest routes. Under this design, the opticaltransponders are often over-engineered for optical transmission atshorter distances because the signal degradation effects on the WDMsignals are less in such optical transmission and a data rate perwavelength higher than that for the long-distance transmission couldhave been used in the short-distance transmission to achieve the same orcomparable data error rate. Another way to deploy the optical DWDMtransponders is to deploy different optical transmitters optimized fordifferent reach ranges or data bit rates at each optical transponderwhich selects a suitable optical transmitter based on the requirementsof the optical transmission. This design can require devicequalification, test, deployment, training and sparing of multipledifferent transponder card variants and may be undesirable for serviceproviders.

The transmission condition of a particular optical path link in theoptical system can be measured with various parameters. The opticalsignal to noise ratio of the optical transmission link can be used torepresent the transmission condition and thus can used to select theproper M-QAM constellation for the signal modulation. The data bit errorrate in the optical transmission link can also be used to represent thetransmission condition and thus can be used to select the proper M-QAMconstellation for the signal modulation. The data bit rate perwavelength or the spectral efficiency in the optical channel signal canalso be used to represent the transmission condition and thus can beused to select the proper M-QAM constellation for the signal modulation.As yet another example, a least mean square error calculator can beimplemented in the digital signal processor to calculate the least meansquare value of the data bit error to represent the channel quality orfidelity and can be used to select the proper M-QAM constellation forthe signal modulation. In addition, combinations of these differentparameters can be used to select the proper M-QAM constellation for thesignal modulation.

The present adjustable rate M-QAM programmable modulation transpondersbased on programmable signal modulation can be deployed in such anoptical transport network to select the signal modulation format with adesired bit rate for each transponder to meet the specific reachrequirement and OSNR of each DWDM channel. This adjustable bit rateoptical transmission using programmable signal modulation at eachoptical transmitter can be used to maximize the spectral efficiency foreach wavelength and maximize the capacity of the fiber optic cable. Asingle transponder card type can be used based on the present adjustablebit rate optical transmission using programmable signal modulation ateach optical transponder and the associated sparing cost can thus beminimized by having only one card flavor per sparing depot. The baudrate of the channel can be kept constant and it's maximum limit can beset by the available optical bandwidth of each DWDM channel. Differentlevels of M-QAM can be provided to code a certain number of symbols perbaud to maximize the data rate, keeping the spectral width of the signalconstant, at the optimum maximum value. In some system implementations,a WDM signal may be required to propagate through at least 5 ROADMexpress filters, with typical cascaded optical filter FWHM bandwidth of20 to 25 GHz. The present adjustable bit rate optical transmission usingprogrammable signal modulation can be used to meet such and otherrequirements for each transmitter.

FIG. 1C illustrate an exemplary adjustable bit rate optical transmitterusing programmable signal modulation and an exemplary optical receiver,respectively. This example shows the basic functional block diagrams foran optical coherent detection modulation scheme, with control of theamplitude of both in-phase, I and quadrature phase, Q, components of themodulated signal. The adjustable bit rate optical transmitter includes adigital signal processing (DSP) unit 101 and accompanying digital toanalog conversion (DAC) circuitry to drive different programmableMary-Quadrature Amplitude Modulation (M-QAM) schemes. The DSP 101 isprogrammed to apply a control algorithm to select a proper QAM schemefrom the multiple QAM schemes for the signal modulation of the opticaltransmitter. In this particular example, the nominal baud rate, b_(n),can be set as a constant and thus the optical transmission bandwidth foroptical WDM signals under the different M-QAM constellations can remainessentially unchanged. Under this fixed baud rate configuration, thedata bit rates and the data spectral efficiency can be increased withoutchanging or modifying the optical network infrastructures. As such, anoptical system can be upgraded to a system capable of higher bit datarates and higher data spectral efficiency by installing the presentadjustable bit rate optical transmitters and respective opticalreceivers without changing the existing system fiber networkinfrastructures. In other implementations, the baud rates in theadjustable bit rate optical transmitter may also be adjustable to offermore continuous rate adaption rather than the discrete steps from movingbetween M-QAM symbol spaces.

In the exemplary adjustable bit rate optical transmitter in FIG. 1C, aCW laser 112 is used to produce a CW laser beam to an optical splitter113 which splits the CW laser beam into two laser beams for carrying theI-phase modulation signal and the Q-phase modulation signal,respectively. A first optical Mach-Zehnder modulator 114A is used tomodulate the first CW laser beam to carry the I-phase modulation signalbased on a selected QAM constellation and a second optical Mach-Zehndermodulator 114B is used to modulate the second CW laser beam to carry theQ-phase modulation signal based on the selected QAM constellation. Themodulated beams out of the two optical modulators 114A and 114B arephase shifted by π/2 (90 degrees) by a phase shifter 115 and arecombined by an optical coupler 116 to form the output optical WDMchannel for transmission.

The example in FIG. 1C uses only one optical polarization.Alternatively, two orthogonal optical polarizations can be utilized in apolarization multiplexing (PM) configuration to double the trafficcarrying capacity by using a separate set of two quadrature modulatorsfor each orthogonal polarization state and a polarization beam combinerto combine the two orthogonally polarized optical WDM channels at thesame WDM wavelength to produce a PM output signal carrying two differentchannels. The two different optical transmitters may use generated thetwo optical channels with orthogonal polarizations from a common opticalcarrier source at the WDM wavelength. In one implementation, a phaseadjustment is provided in the signal modulation to vary the Baud skewbetween the first and second channels with the orthogonal polarizations.Two different ways for setting the Baud skew may be used: (i) the Baudskew is set at the time of manufacturing (ii) the error rate informationfrom the receiver is provided thru the feedback channel to adjust theBaud skew between the first and second channels. This Baud skew can beused to reduce nonlinear penalties in the transmission link.

FIG. 1C also shows an example of the optical receiver design for theoptical transmitter. This optical receiver design implements a digitalprocessing based synchronous coherent detection scheme that includes afree-running optical Local Oscillator (LO) 132 to produce an opticallocal oscillator signal, and feed forward carrier recovery, polarizationdemultiplexing, compensation for chromatic dispersion (CD) andpolarization mode dispersion (PMD) by using two sets of balancedphotodiodes 133A and 133B to produce two optical analog outputs, ananalog to digital conversion (ADC) block to convert the signals intodigital signals, and digital processing in a receiver DSP unit 134 by anadjustable digital Finite Impulse Response (FIR) filter. Alternatively,optical coherent QAM detection can also be achieved by using an opticallocal oscillator and an optical phase-lock loop (PLL) without usingdigital signal processing.

As an example for implementing the design in FIG. 1B, the baud rate maybe set to a fixed value of 25 Gps. The value for a actual implementationbaud rate may be set by including some overhead for PCS encoding, OTNframing and FEC. Two optical beams in two orthogonal opticalpolarizations can be modulated to carry two different data channelsbased on polarization multiplexing (PM).

FIG. 2 shows examples of eight different QAM modulation constellationsbased on the Polarization Multiplexed Phase Shift Keying (PM-PSK) forthe fixed baud rate of 25 Gps. Examples of ADC and DAC resolutions areshown for implementing the illustrated M-QAM. Other suitable ADC and DACresolutions may be used. To maximize the reach of the optical WDMchannel, the adjustable rate transponder can be configured to transmitPM-PSK as shown in FIG. 2( a). This modulation format can provide themaximum OSNR sensitivity and maximum launch power possible, thereforemaximizing the distance that can be transmitted between signal 3R(reshape, retransmit and retime) regeneration points in the network. Thecapacity in this example would be 50 Gb/s (1 bits/s/Hz). If the specificchannel has excess performance margin, the transponder can bereconfigured to PM-QPSK as shown in FIG. 2( b). The OSNR sensitivity isthe same (I and Q component noise is independent) as in FIG. 2( a) andthe launch power would be slightly lower than PM-PSK as it is moresensitive to nonlinear phase noise (90° between symbol states for QPSK,whereas 180° for PSK). This doubles the capacity to 100 Gb/s. In asimilar fashion, if the channel has still more margin (i.e. typically ifit operates over a shorter reach) the transponder can re-configure asPM-8QAM [FIG. 2( c)], PM-16QAM [FIG. 2( d)], PM-32QAM [FIG. 2( e)],PM-64QAM [FIG. 2( f)], PM-128QAM [FIG. 2( g)], PM-256QAM [FIG. 2( h)],etc.

As illustrated by the constellation diagrams in FIG. 2, each time theM-QAM bits/symbol rate is incremented, the channel carrying capacityincreases, at the expense of an increase in the required OSNR. The OSNRincrease is due to the reduction in the Euclidean distance from symbolto symbol. Rectangular QAM constellations are used in this analysis forimplementation simplicity and circular QAM constellations can also beused to increase the minimum Euclidean distance between adjacentsymbols.

FIG. 3 illustrates the tradeoff for channel bit rate capacity/spectralefficiency versus OSNR for the different PM-QAM modulation formats.

FIG. 4 shows another example of how programmable M-QAM adjustable ratetransponders may work in an actual DWDM system with ROADMs at eachtraffic add/drop terminal. This example shows that shorter links wouldtypically allow higher capacity by using higher number of levels for theM-QAM constellations and that longer optical path circuits transmit lesscapacity by using QAM constellations with lower number of levels, DQPSKor PSK transmission to maximize the reach distance. Other tradeoffs arealso possible with this approach. For example, on high capacity routes,reduced distance between EDFAs or Raman amplification used to boostOSNR, which could then be traded for increased capacity by using ahigher level QAM constellations selected from the available QAMconstellations in the digital processing unit for the transmitter.Another degree of freedom is that the use of 3R regenerators could beused to limit, or reduce, the optical reach on certain wavelengths orgeographic sections of the network so that higher order M-QAM modulationcould be utilized to increase the wavelength channel's transmission ratecapacity, or spectral efficiency. In the example in FIG. 4, the opticaltransmitters for different optical path links can use the same rateadjustable optical transmitters described in this application that areset to operate at different M-QAM constellations with different data bitrates for different communication distances.

In deployment of the present adjustable bit rat optical transmitters, abank of rate adjustable M-QAM transponders could be used in a completeoptical terminal sub-system to maximize the channel data rate on a perwavelength basis. FIG. 5 shows one example of a rate adjustable opticaltransponder that is connected to an ROADM within an optical node in anetwork. This rate adjustable optical transponder includes a bank ofprogrammable rate adjustable optical transmitters at different tunableoptical WDM wavelengths based on tunable laser diodes and a bank ofoptical receivers with tunable optical local oscillators (LOs) forwavelength selectivity in detection. The design in the example in FIG. 5can be used to maximize the total fiber capacity, by adapting the bitrate each wavelength channel (typically 80 independent channels oncommercial DWDM systems), dependent on the channel distance andinstantaneous performance of each channel path, using pre-FEC BERfeedback to adapt the M-QAM constellation on a real-time basis. Oneembodiment could be that this optical terminal resides within anInternet Protocol (IP) router, where IP packets can be efficientlystatistically multiplexed into the bank of rate adjustable DWDMtransponders, maximizing IP data throughput. Notably, the rateadjustable optical transponder in FIG. 5 can be connected to the thirdparty ROADM system without modifying the ROADM and other aspects of thesystem. This feature reduces cost and labor in upgrading the system.

In implementing the present adjustable bit rate optical transmissionusing programmable signal modulation, various mechanisms can be used toselect a suitable modulation format with a suitable data bit rate for agiven optical transmitter. In one implementation, for example, designrules form optical link engineering tools can be used to determine apriori what M-QAM signal constellation be programmed for each specifictransmission route and a look-up table for the transmission routs andthe modulation formats is generated and stored in the control for theoptical transmitter. In operation, the routing information is used toselect the proper modulation format from the look-up table to controlthe modulation. When the routing is changed for the optical WDM channelof that optical transmitter, a different modulation format is selectedfrom the look-up table for the optical transmitter.

FIG. 6 illustrates one example of this process. First, the optical pathproperties of different optical link paths of different optical WDMchannels generated by a programmable rate adjustable optical transponderin an optical network are obtained based on associated engineeringdesign rules used in designing these optical link paths. Next,appropriate M-QAM constellations for signal modulations in the differentoptical WDM channels are selected for the different optical link paths,respectively, based on the obtained path properties (e.g., link pathlengths, OSNR, BER). This mapping between the appropriate M-QAMconstellations for signal modulations and the optical link paths is usedto form the look-up table. In operation of the system, this look-uptable is used to control each of programmable adjustable bit ratetransmitters in the optical transponder for each optical WDM channel tooperate in a corresponding M-QAM constellation selected from multipleavailable M-QAM constellations to generate a corresponding optical WDMchannel.

In another implementation of the adjustable bit rate opticaltransmission using programmable signal modulation, the adjustable ratetransponders can be deployed in the field, then go through aself-training “set and forget” procedure at channel turn-up thatdetermines the maximum usable capacity. This implementation can use thepre-FEC BER (Q margin to FEC threshold) as a feedback figure of merit todetermine the maximum transmission rate, with an adequate margin forknown performance fading in the channel. FIG. 7 illustrates one examplefor this procedure. First, one or more programmable adjustable bit rateoptical transponders are installed in an optical network. The installedtransponder is operated to run a tune-up test to measure path propertiesof different optical link paths of different optical WDM channelsgenerated by the transponder in the optical network. Respective M-QAMconstellations for signal modulations in the different optical WDMchannels are then selected for the different optical link paths,respectively, based on the measured path properties (e.g., link pathlengths, OSNR, and BER). Each of the programmable adjustable bit ratetransmitters in the optical transponder for each optical WDM channel iscontrolled to operate in a corresponding M-QAM constellation selectedfrom multiple available M-QAM constellations to generate a correspondingoptical WDM channel. In practical implementations, the transmissioncondition of an optical WDM channel signal can change over time and thusa selected QAM constellation may become unsuited at a later time. Hence,after a period of operation in the network, the tune-up test may beperformed again to update the measured path properties for each linkpath and to update the selection of the M-QAM constellation for eachchannel.

In yet another implementation, a dynamic feedback mechanism is providedto inform an adjustable bit rate optical transmitter of the currentoptical transmission requirement or condition of the link so that thebit rate adapts over time continuously to the instantaneous performanceof the link. This feedback provides a rate adaptive use of the rateadjustable transmitters.

FIG. 8 shows an example of this feedback control process in adjusting aprogrammable adjustable bit rate optical transmitter. During operationof a programmable adjustable bit rate optical transponder, the opticalperformance of different optical link paths of different optical WDMchannels generated by the transponder at respective optical receivers inthe network is monitored. This monitoring can be achieved at an opticalreceiver, for example. An optical communication signal can be used as afeedback to communicate monitored optical performance from each opticalreceiver to the optical transponder. The optical transponder is operatedto analyze the monitored performance of different optical link paths ofdifferent optical WDM channels to determine link performance for anM-QAM constellation currently used in each programmable adjustable bitrate transmitter. Based on the link performance, the programmableadjustable bit rate transmitter is controlled to change to a differentM-QAM constellation when the measured link performance and the M-QAMconstellation currently in use do not match. When the measured linkperformance and the M-QAM constellation currently in use match, theprogrammable adjustable bit rate transmitter is controlled to maintainthe M-QAM constellation currently in use. This feedback and control canbe implemented in real time during the normal operation where the aboveprocessing steps are repeated.

In one implementation, the pre-FEC BER thresholding may be used as afeedback mechanism to control the selection of a M-QAM constellation forthe signal modulation, and hence the bit rate associated with theselected M-QAM constellation. This implementation can dynamicallymaximize the transmission capacity of the optical fiber on a per channelbasis. This design can improve the performance of DWDM channels invarious existing systems where many channels are operated below theirperformance full capacity, either through conservative design and/oroperation over shorter links.

FIG. 9 shows an example of an optical communication system based on theadaptive bit rate optical transmission 110 using programmable signalmodulation and dynamic feedback. A feedback signal, e.g., an opticalfeedback signal 920, is generated at an optical receiver 130 in adestination optical transponder and is directed back to the opticaltransmitter to control the signal modulation format for the opticaltransmitter. Various performance monitoring mechanisms can be used as afeedback mechanism to control (and maximize) the bit rate. Theperformance feedback signal can be communicated from the receiver(tail-end) to the transmitter (head-end) where the bit rate (and M-QAMconstellation) will be selected to maximize bit rate, without causingany post FEC errors (error-free transmission).

As an example, the OSNR value can be one receiver performance monitorparameter for feedback. One limitation with using OSNR is that it onlyloosely correlates with the actual link performance and it does notinclude any eye distortion effects (such CD, PMD, SPM) that also impactsthe transmission performance. Various commercial core optical networkingequipment uses forward error correction (FEC) encoding to improve thereach performance and this parameter can be used for the feedback tocontrol the present adjustable bit rate optical transmitter. A usefulbenefit of FEC is that the pre-FEC BER can also be monitored. Fromknowledge of the FEC coding gain (determined by the specific algorithmchosen) the system margin (typically given in dBQ) can be easilyderived. The optimum M-QAM constellation, and hence transmitted bitrate, can yield a margin value that is high enough not to cause anypost-FEC output bit errors due to fast transient effects (that theadjustable bit rate transponder will not be fast enough to track) butnot so large that the transponder is transmitting at too low a bit rateand not maximizing throughput performance. Transport networks, such asSONET and OTN, offer in-band communications channels that can be used tosend supervisory data, typically in the overhead bytes of the signalframe.

FIG. 10 shows an implementation of the feedback in FIG. 9 where theGeneral Communication Channel (GCC) optical signal under ITU StandardG.709 for OTN systems is used to send the pre-FEC BER feedback signalfrom the tail-end optical transponder to the head-end opticaltransponder via the datapath in the opposite direction of thetransmitted optical WDM channel. As illustrated, the GCC transmitter inthe head-end optical transponder operates the GCC transmitter tomodulate the received FEC information from the receiver RX onto the GCCsignal to the M-QAM tunable bit rate transmitter. The head-endtransponder then terminates the GCC and, based on the pre-FEC BER data,determines the optimum M-QAM constellation with a respective bit rate tobe used for the M-QAM tunable bit rate optical transmitter. The selectedM-QAM constellation is then used to control the signal modulation in theM-QAM tunable bit rate optical transmitter. Other optical signals canalso be used for the optical feedback 920 in selecting and controllingthe QAM constellations.

As an example, FIG. 11 shows how HIGH and LOW thresholds on theinstantaneous margin derived from the Rx end pre-FEC BER data can beused to send interrupt signals to either increase or decrease the M-QAMconstellation for an optical transmitter, respectively, to maximize thebit rate throughput. A pre-determined LOW threshold can be set and thusa trigger can be sent if the margin drops below the threshold. Thetrigger could then cause an interrupt signal which could either directlyreduce the data rate (and M-QAM constellation) or more likely it wouldsend a signal to the controller of the data flow (e.g. flow controlmechanism in an IP router, or bandwidth aggregation control in acrossconnect switch). The flow controller could then take theappropriate action (e.g. re-route some traffic by a different path orwavelength) so that when the bit rate is throttled back, there is noimpact to the end user communication. Some buffering may also be neededduring the time period over which the channel re-adjusts to the newM-QAM constellation and synchs up, at both transmit and receive ends.When the margin exceeds some specified HIGH threshold, then effectivelythis channel is under-utilized and the bit rate (M-QAM constellation)can be increased to take advantage of this improved link performance andmaximize bit rate. Again, crossing this threshold could instantaneouslytrigger the bit rate increase, but it may be more useful to send aninterrupt signal to the bandwidth flow controller to say that we canincrease the bit rate of the channel and allow it to take any necessaryaction (buffering, re-route traffic flows, etc.) before the bit rateincrease occurs on the line.

Various features described in this application can be used to operate asingle adjustable bit rate transponder to modify its bit rate viachanging the M-QAM constellation transmission to maximize the achievabledata throughput on any particular wavelength channel in a DWDM system.The actual channel performance depends various factors including thereach (OSNR) and the accumulated signal distortions from fiber lineareffects (such as CD, PMD and PDL) and nonlinear optical effects such asself phase modulation (SPM), inter-bit four wave mixing (IFWM), crossphase modulation (XPM), and four wave mixing (FWM), many of which causetemporal variation, or fading in the channel. By assuming a maximum baudrate that is selected to ensure propagation through the host DWDM systemoptical filters (e.g. 25 Gbaud rate ensures transmission throughmultiple cascaded 50 GHz ROADMs) then increasing the bit rate of theadjustable transponder without changing the baud rate (or opticalspectral width, to the first order) means that the signal will stillpropagate through the DWDM filters, with no changes required to theinstalled base of DWDM equipment.

Different optical transmitters at different WDM wavelengths traversingdifferent distances in a system can be operated to adjust theirrespective levels in the M-QAM constellation to trade the spectralefficiency for reach. Short reach demands can be served with a M-QAMconstellation with a relatively high bit rate, whereas longer circuitswill optimize for reach and have a subsequently lower bit rate. As onlyone single flavor of adjustable rate transponder is needed, there is nota high penalty associated with sparing compared to the case when youhave multiple different fixed rate transponders with different reachcapabilities, in which case each sparing depot must store one of eachtype of transponder. The cost of the adjustable rate transponder designis dominated by the electro-optics and the cost structure for theadjustable design would be approximately the same as a fixed transpondersupporting PM-QPSK. This means that increasing the data rate for shorterreach circuits comes at a very low cost premium (need a DAC at thetransmit end and higher resolution ADC at the receiver), which isattractive to service providers.

The present adjustable bit rate optical transmission using programmablesignal modulation can also be implemented to achieve considerable spaceand power advantages. When the transponder transmits at a higher daterate, the power consumption and transponder footprint do not change, sothis realizes considerable operational expense (OPEX) savings to theservice provider and reduces the carbon footprint of the serviceprovider backbone optical transport network, with positive environmentalimpact.

The present adjustable bit rate optical transmission 110 usingprogrammable signal modulation can be used to provide continuous,adaptive bit rate control so that the channel can be maintained at itsthe maximum data carrying capacity at any point in time. As manydistortion effects are temporally varying (such as PMD, PDL, XPM andFWM) the performance margin will also fluctuate. Various other opticalsystems with fixed rate transponders design can use an added margin tocope with worst-case values for each of these effects. In such fixedrate systems, at any typical point in time, the channel normally hasexcess performance margin as these effects are not at worst-case valuesat that particular point in time. The present adaptive bit rate opticaltransmission 110 using programmable signal modulation can be implementedto allow the channel to adapt and maximize the bit rate in real time,depending on the value of instantaneous temporal distortions that aresubject to fading phenomena. Adjustable bit rate transmission providesadditional value in the network by increasing the average bit ratetransmitted over time in the optical transport network. It also reducesthe risk that too many fixed rate transponders or regenerators have beendeployed in a network by overly conservative link engineeringassumptions.

For longer reach demands, it is envisaged that typically a low M-QAMconstellation (possibly [PM]-QPSK for maximum reach) will be selected bythe adjustable rate transponder. However, if the carrier values spectralefficiency (for example if they have a low # optical fibers available,or want to maximize capacity on an existing DWDM system) the serviceprovider has the option of deploying rate adjustable regenerators alongthe link. This reduces the required OSNR sensitivity and hence shouldallow transmission of a higher data rate from the adjustable ratetransponder and regen(s) on the link. The tradeoff is higher spectralefficiency (more capacity per fiber) vs. regenerator(s) cost. If aregenerator(s) is/are used in the link, then the whole end-to-end linkwill have the bit rate limited to the lowest margin OEO section. Assuch, communication will be needed between the regens and end terminaltransponders to make sure that the adjustable bit rate is the same forall sections and dictated by the lowest margin OEO section. Again,margin measurement using pre-FEC BER monitoring and in-bandcommunications channel (such as the GCC) can be used for this purposeand control.

While this specification contains many specifics, these should not beconstrued as limitations on the scope of an invention or of what may beclaimed, but rather as descriptions of features specific to particularembodiments of the invention. Certain features that are described inthis specification in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination can in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or a variation of a subcombination.

Only a few implementations are disclosed. However, it is understood thatvariations and enhancements may be made.

1. A method for optical communications, the method comprising: operatinga programmable optical transmitter to provide a plurality of differentquadrature amplitude modulation (QAM) constellations with different databit rates in controlling signal modulation of an optical channel signalwith a variable data bit rate selected from the QAM constellations;operating the optical transmitter to select one of the QAMconstellations to control the signal modulation based on a condition ofan optical transmission link that transmits the optical channel signal;and operating the optical transmitter to select another one of the QAMconstellations to control the signal modulation when the condition ofthe optical transmission link changes, wherein after a period of time inoperating the optical transmitter under the selected QAM constellation,the method comprises operating the optical transmitter to performanother test to measure performance of the optical transmission link;using measured performance of the optical link to determine whether theselected QAM constellation currently in use for the signal modulation inthe optical transmitter is adequate; and operating the opticaltransmitter to select a different QAM constellation in transmitting theoptical channel signal over the optical transmission link when thepreviously selected QAM constellation currently in use for the signalmodulation is not adequate based on the measured performance.
 2. Themethod as in claim 1, wherein: the change of the condition of theoptical transmission link includes a distance of the opticaltransmission link.
 3. The method as in claim 2, comprising: operatingthe optical transmitter to change the QAM constellation for signalmodulation to a lower order QAM constellation with a lower data bit ratewhen a length of the optical transmission link increases.
 4. The methodas in claim 1, wherein: the change of the condition of the opticaltransmission link includes an optical signal to noise ratio of theoptical transmission link.
 5. The method as in claim 1, wherein: thechange of the condition of the optical transmission link includes a databit error rate in the optical transmission link.
 6. The method as inclaim 1, wherein: the change of the condition of the opticaltransmission link includes a data bit rate per wavelength in the opticalchannel signal.
 7. The method as in claim 1, comprising: using a leastmean square value of a data bit error rate in the optical transmissionlink to indicate the change of the condition of the optical transmissionlink.
 8. The method as in claim 1, wherein: the QAM constellationsinclude circular QAM constellations.
 9. The method as in claim 1,wherein: the QAM constellations include square QAM constellations. 10.The method as in claim 1, comprising: providing a pre-determined look-uptable for the QAM constellations and operating conditions of the opticaltransmission link for transmitting the optical channel signal; and usinga given operating condition of the optical transmission link to select aQAM constellation for the signal modulation in the optical transmitterfrom the pre-determined look-up table.
 11. The method as in claim 1,comprising: during operation of the optical transmitter in transmittingthe optical channel signal over the optical transmission link,monitoring the condition of the optical transmission link; using afeedback signal to communicate the monitored condition to the opticaltransmitter; and operating the optical transmitter to select a QAMconstellation for the signal modulation in the optical transmitter inresponse to the feedback signal.
 12. The method as in claim 1, wherein:the different QAM constellations with different data bit rates in theprogrammable optical transmitter have a fixed baud rate.
 13. The methodas in claim 1, comprising: changing a baud rate when changing from afirst of the different QAM constellations with different data bit ratesin the programmable optical transmitter to a second of the different QAMconstellations.
 14. The method as in claim 1, comprising: operating acoherent QAM detection optical receiver to receive the optical channelsignal from the optical transmission link and to extract data from thereceived optical channel signal.
 15. The method as in claim 14, wherein:the coherent QAM detection optical receiver includes a local opticaloscillator that produces an optical signal to mix with the receivedoptical channel signal in extracting data from the received opticalchannel signal.
 16. The method as in claim 14, wherein: the coherent QAMdetection optical receiver includes a digital signal processor thatperforms a digital coherent QAM processing operation in extracting datafrom the received optical channel signal.
 17. A method for opticalcommunications, the method comprising: operating a programmable opticaltransmitter to provide a plurality of different quadrature amplitudemodulation (QAM) constellations with different data bit rates incontrolling signal modulation of an optical channel signal with avariable data bit rate selected from the QAM constellations; operatingthe optical transmitter to select one of the QAM constellations tocontrol the signal modulation based on a condition of an opticaltransmission link that transmits the optical channel signal; andoperating the optical transmitter to select another one of the QAMconstellations to control the signal modulation when the condition ofthe optical transmission link changes, wherein prior to operating theoptical transmitter in transmitting the optical channel signal over theoptical transmission link, the method comprises: operating the opticaltransmitter to perform a test to measure performance of the opticaltransmission link; using measured performance of the optical link toselect a QAM constellation for the signal modulation in the opticaltransmitter; and operating the optical transmitter under the selectedQAM constellation in transmitting the optical channel signal over theoptical transmission link.
 18. The method as in claim 17, comprising:after a period of time in operating the optical transmitter under theselected QAM constellation, operating the optical transmitter to performanother test to measure performance of the optical transmission link;using measured performance of the optical link to determine whether theselected QAM constellation currently in use for the signal modulation inthe optical transmitter is adequate; and operating the opticaltransmitter to select a different QAM constellation in transmitting theoptical channel signal over the optical transmission link when thepreviously selected QAM constellation currently in use for the signalmodulation is not adequate based on the measured performance.
 19. Asystem for optical communications, the system comprising wherein: anoptical transponder including: a plurality of programmable opticaltransmitters to produce optical WDM channel signals at different opticalWDM wavelengths, each programmable optical transmitter of the pluralityof the programmable optical transmitters comprising a digital signalprocessing unit that is programmed to include a plurality of differentquadrature amplitude modulation (QAM) constellations with different databit rates in controlling signal modulation of an optical WDM channelsignal with a variable data bit rate selected from the QAMconstellations, wherein the optical WDM channel signal at the WDMchannel wavelength from the programmable optical transmitter is in afirst optical polarization; a second programmable optical transmitterthat produces a second optical WDM channel signal at the WDM channelwavelength in a second optical polarization that is orthogonal to thefirst optical polarization, and a polarization combiner that combinesthe optical WDM channel signal in the first optical polarization and thesecond optical WDM channel signal in the second optical polarization toproduce a polarization multiplexed signal for transmission to an opticaltransmission network connected to optical transponder to transmit theoptical WDM channel signals; at least one optical receiver in theoptical transmission network to receive at least one of the optical WDMchannel signals from the optical transponder and to include a coherentQAM detection mechanism to extract data from the received optical WDMchannel signal; and a feedback mechanism in the optical transmissionnetwork to communicate to the optical transponder a feedback signalindicative of a condition of an optical transmission link that transmitsthe at least one optical WDM channel signal form the optical transponderto the optical receiver, wherein the optical transponder responds to thefeedback signal to select one QAM constellation from the QAMconstellations to control the signal modulation in a respectiveprogrammable optical transmitter based on the feedback signal andselects another QAM constellation to control the signal modulation inthe respective programmable optical transmitter when the condition ofthe optical transmission link changes.
 20. A method for opticalcommunications, the method comprising: connecting a programmable opticaltransponder in an optical communication network, each programmableoptical transponder comprising a plurality of programmable opticaltransmitters to produce optical WDM channel signals at different opticalWDM wavelengths and a plurality of optical receivers for detectingoptical WDM channel signals, each programmable optical transmittercomprising a digital signal processing unit that is programmed toinclude a plurality of different quadrature amplitude modulation (QAM)constellations with different data bit rates in controlling signalmodulation of an optical WDM channel signal with a variable data bitrate selected from the plurality of the QAM constellations; obtainingperformance information for each optical path link for each of theoptical WDM channel signals produced by a programmable opticaltransponder; operating each of the programmable optical transmitters ineach optical transponder under a selected QAM constellation that isselected from the plurality of the QAM constellations in the digitalprocessing unit of the programmable optical transmitter based on theperformance information for the respective optical path link; providinga feedback mechanism in the optical network to communicate to theoptical transponder a feedback signal indicative of a change of theperformance of the optical path link for each optical WDM channel signalfrom a respective programmable optical transmitter; and operating aplurality of programmable optical transmitters to change a selected QAMconstellation currently in use to a different QAM constellation when therespective change of the performance of the optical path link meets apre-determined condition for changing the QAM constellation, whereinsaid operating the plurality of programmable optical transmittersincludes operating a first programmable optical transmitter in theprogrammable optical transponder under a first QAM constellation at afirst data rate in transmitting a first optical WDM channel signal alonga first optical path link in the optical network; and operating a secondprogrammable optical transmitter in the programmable optical transponderunder a second QAM constellation at a second data rate lower than thefirst data rate in transmitting a second optical WDM channel signalalong a second optical path link that is longer than the first opticalpath link.
 21. A method for optical communications, the methodcomprising: connecting a programmable optical transponder in an opticalcommunication network, each programmable optical transponder comprisinga plurality of programmable optical transmitters to produce optical WDMchannel signals at different optical WDM wavelengths and a plurality ofoptical receivers for detecting optical WDM channel signals, eachprogrammable optical transmitter comprising a digital signal processingunit that is programmed to include a plurality of different quadratureamplitude modulation (QAM) constellations with different data bit ratesin controlling signal modulation of an optical WDM channel signal with avariable data bit rate selected from the plurality of the QAMconstellations; obtaining performance information for each optical pathlink for each of the optical WDM channel signals produced by aprogrammable optical transponder; operating each of the programmableoptical transmitters in each optical transponder under a selected QAMconstellation that is selected from the plurality of the QAMconstellations in the digital processing unit of the programmableoptical transmitter based on the performance information for therespective optical path link; providing a feedback mechanism in theoptical network to communicate to the optical transponder a feedbacksignal indicative of a change of the performance of the optical pathlink for each optical WDM channel signal from a respective programmableoptical transmitter; and operating a plurality of programmable opticaltransmitters to change a selected QAM constellation currently in use toa different QAM constellation when the respective change of theperformance of the optical path link meets a pre-determined conditionfor chan in the QAM constellation, wherein said operating the pluralityof programmable optical transmitters includes operating a firstprogrammable optical transmitter in the programmable optical transponderunder a first QAM constellation at a first data rate in transmitting afirst optical WDM channel signal along a first optical path link in theoptical network; and operating a second programmable optical transmitterin the programmable optical transponder under a second QAM constellationat a second data rate lower than the first data rate in transmitting asecond optical WDM channel signal along a second optical path link thathas a higher optical signal to noise ratio than the first optical pathlink.
 22. The method as in claim 21, wherein: the pre-determinedcondition for changing the QAM constellation includes a distance of theoptical path link.
 23. The method as in claim 21, wherein: thepre-determined condition for changing the QAM constellation includes anoptical signal to noise ratio in the optical path link.
 24. The methodas in claim 21, wherein: the pre-determined condition for changing theQAM constellation includes a data bit error rate in the optical pathlink.
 25. The method as in claim 21, wherein: the pre-determinedcondition for changing the QAM constellation includes a data bit rateper wavelength in the optical channel signal.
 26. The method as in claim21, comprising: using a least mean square value of a data bit error ratein the optical transmission link to indicate pre-determined conditionfor changing the QAM constellation.
 27. The method as in claim 21,wherein: configuring each programmable optical transmitter to include2-QAM, 4-QAM, 8-QAM, 16-QAM, 32-QAM, 64-QAM, 128-QAM and 256-QAM in theplurality of the QAM constellations.
 28. The method as in claim 21,comprising: applying a coherent QAM detection scheme in detecting eachoptical WDM channel signal under a selected QAM constellation in theoptical network.
 29. The method as in claim 21, comprising: maintainingthe plurality of the QAM constellations to be at a fixed common baudrate.
 30. A method for optical communications, the method comprising:providing a plurality of programmable optical transmitters in an opticalnode in a network, each programmable optical transmitter comprising aplurality of different quadrature amplitude modulation (QAM)constellations with different data bit rates and operating to controlsignal modulation of an optical channel signal with a variable data bitrate selected from the QAM constellations; determining opticaltransmission performance of optical path links for transmitting opticalchannel signals from the optical transmitters in the optical node,respectively, based on at least one of an optical path link length andan optical signal to noise ratio for each of the optical path links toselect one of the QAM constellations for each optical transmitter tocontrol the signal modulation, wherein the determining of the opticaltransmission performance of optical path links is performed by:performing a test on each optical path link to measure the opticaltransmission performance; and using the measured optical transmissionperformance in the test to select a QAM constellation for the opticalpath link; and operating the optical transmitters in the optical nodeunder the selected QAM constellations with different data bit rates. 31.The method as in claim 30, comprising: determining the opticaltransmission performance of optical path links based on engineeringdesign rules for designing the optical path links.
 32. A method foroptical communications, the method comprising: providing a plurality ofprogrammable optical transmitters in an optical node in a network, eachprogrammable optical transmitter comprising a plurality of differentquadrature amplitude modulation (QAM) constellations with different databit rates and operating to control signal modulation of an opticalchannel signal with a variable data bit rate selected from the QAMconstellations; determining optical transmission performance of opticalpath links for transmitting optical channel signals from the opticaltransmitters in the optical node, respectively, based on at least one ofan optical path link length and an optical signal to noise ratio foreach of the optical path links to select one of the QAM constellationsfor each optical transmitter to control the signal modulation, whereinthe determining of the optical transmission performance of optical pathlinks is performed by: measuring the optical transmission performance ata receiver end of each optical path link; operating a feedback signal toinform a respective optical transmitter of the measured opticaltransmission performance at the receiver end; and using the measuredoptical transmission performance in the feedback signal to operate theoptical transmitter in selecting a QAM constellation for the opticalpath link; and operating the optical transmitters in the optical nodeunder the selected QAM constellations with different data bit rates.